\begin{tikzpicture}[>=Stealth, scale=0.5,
    every node/.style={fill=white, inner sep=1pt},
]
    \draw [->] (-3, 0) -- (3, 0) node[below=0.2em] {$x$};
    \draw [->] (0, -3) -- (0, 3) node[left=0.2em]  {$y$};
    \draw (0, 0) node [below right] {\small $O$};

    \pgfmathsetmacro{\k}{0.8}
    \pgfmathsetmacro{\b}{1.2}
    \draw[very thick, domain=-2.5:2.0, samples=5] plot (\x, {\k*\x + \b});
    \draw[decorate,decoration={brace,mirror,amplitude=0.2cm}] (0, 0) -- (0, \b)
                node [pos=0.8, right=0.5em] {\small $b > 0$};
    \draw[very thick, domain=-1.5:3.0, samples=5] plot (\x, {\k*\x - \b});
    \draw[decorate,decoration={brace,mirror,amplitude=0.2cm}] (0, 0) -- (0, -\b)
                node [pos=0.8, left=0.5em] {\small $b < 0$};
\end{tikzpicture}

